h-BN has a structure resembling that of graphite and comprising planar reticular hexagonal B-N arrangements as stacked, and is known as a compound which is outstanding in characteristics such as thermal conductivity, electrical insulating properties, heat resistance, corrosion resistance, chemical stability and lubricity. Active efforts are directed to the development of uses of this compound. To utilize these characteristics, h-BN is used in various fields in the form of a powder as a solid lubricant, heat-resistant parting agent and material for cubic BN (c-BN), or in the form of sintered bodies prepared from a powder thereof and serving as melting crucibles, electric insulating materials and electronic materials.
Attention has been directed in recent years to the heat resistance and heat dissipating property of the compound in the field of electric and electronic components, and further applications are expected to heat sink plates, for example, for use in computers.
The industrial processes already known for producing boron nitride includes: 1) process for nitriding a boron oxide by reduction, 2) process for nitriding elemental boron, and 3) process for nitriding a boron halide by reduction. The h-BN obtained by these processes has a scalelike form attributable to the particular process and is therefore subject to limitations when to be made into sintered bodies or into composite materials in combination with other ceramics or when to be used in the form of other composite materials. More specifically, the use of the scalelike h-BN as a material for sintered bodies inevitably involves formation of voids and consequently encounters difficulty in giving a compacted sintered body. The nitride has the drawback of giving rise to a similar problem also when to be made into composite materials along with other ceramics or resins.
On the other hand, JP-A-151202/1985 discloses a process for producing a boron nitride. According to the publication, a boron nitride in a scalelike, columnar or acicular form can be obtained from at least one boron compound selected from among boric acids and metal salts of boric acids, and at least one nitrogen-containing compound capable of reacting with the boron compound, by forming a compound first wherein boron and nitrogen atoms are conjointly present, and subsequently heating the resulting compound in an inert gas or reducing gas atmosphere at a temperature of at least 600.degree. C. However the fibrous boron nitride available by this process is limited to not greater than 20 .mu.m in fiber length if longest and still remains to be fully improved in size for use as a material for giving improved thermal conductivity.
The boron nitride obtained by this process is not in the form of fine particles of high quality on the order of nanometers, and can not be made into fine crystals of the order of nanometers even if pulverized.
A first object of the present invention is to provide a polycrystalline h-BN compound comprising fine crystals of the order of nanometers or a polycrystalline h-BN compound having a fibrous form, and a process for producing such a compound.